The present invention relates to a magnetic recording medium using a ferromagnetic metal thin film, and more particularly, to a magnetic recording medium having excellent electromagnetic transducing properties, and a large capacity magnetic recording and reproducing apparatus.
For improving the recording density, increasing the output, and reducing the noise of magnetic recording media, it is essential to micronize magnetic particles in the case of a coated medium and crystal grains in the case of a thin film medium. Regarding a medium using metal particles that has heretofore been studied, for example, micronization has progressed and high-performance tapes such as Hi-8 (8-mm high-density magnetic tapes) using extra-fine particles having a cylinder major axis length of approximately 200 nm and a cylinder diameter of approximately 30 nm are now put to practical use. Incidentally, a plurality of particles are subjected to magnetic reversal in a group and signals are recorded when magnetic particles have been formed into a cluster agglomerate or when the interaction between crystal grains is strong even though the magnetic particles or crystal grains of a magnetic medium are extremely fine. When the plurality of particles are subjected to magnetic reversal and when the magnetic reversal unit becomes larger, noise increases at the time of reproducing data. In consequence, the density improvement is greatly hampered.
The size of the magnetic reversal unit is relevant to magnetic viscosity. In other words, it is considered that the greater the fluctuation field of magnetic viscosity becomes, the smaller the magnetic reversal unit is. A description has been given of a meaning of the fluctuation field of magnetic viscosity in Journal of Physics F: Metal Physics, Vol. 14, 1984, pp. L155-L159. Further, a detailed description has also been given of the measurement conditions in Journal of Magnetism and Magnetic Materials, Vol. 127, 1993, pp, 233-240. The principle of measuring the fluctuation field of magnetic viscosity will subsequently be described.
When a new magnetic field is applied to a magnetic material, the magnetization I(t) often varies in relation to the logarithm ln(t) of the field applied time:
I(t)=const+Sxc2x7ln(t)xe2x80x83xe2x80x83(1)
In this case, I(t) represents a magnetic moment per unit volume, and t represents elapsed time after the new magnetic field is applied. The viscosity coefficient S has a positive value when the magnetic field is shifted in the positive direction and has a negative value when the magnetic field is shifted in the negative direction. Moreover, it is known that S can be expressed by the product of the irreversible susceptibility Xirr and the fluctuation field Hf. In other words, there is established the following relation:
S=Xirrxc2x7Hfxe2x80x83xe2x80x83(2)
Therefore, the fluctuation field is determined if S and Xirr are found experimentally. The fluctuation field is a quantity representing the degree of the influence of thermal fluctuation, and a greater fluctuation field signifies that it is easily affected by thermal fluctuation and that the magnetic reversal unit is small in size.
The fluctuation field where the field strength is equal to coercivity or remanence coercivity can also be found from the dependence on the field applied time of the coercivity Hc or remanence coercivity Hr. The coercivity or remanence coercivity, together with field applied time t, often decreases according to the following relation
Hc(or Hr)=xe2x88x92Axc2x7ln(t)+constxe2x80x83xe2x80x83(3)
as the application time elapses. All the specimens mentioned in the present specification satisfied Eq. (3). When the coercivity or remanence coercivity varies with the field applied time t according to Eq. (3), it is known that A takes substantially the same value as that of the fluctuation field Hf where the field strength is equal to the coercivity or remanence coercivity. This procedure is not only simple but also excellent in reproducibility. Hence, the value A is taken as the fluctuation field of magnetic viscosity according to the present invention.
By measurement at room temperature, the fluctuation field thus found has the nature of becoming large in proportion to the absolute temperature at the time of measurement. When a fluctuation field is measured at room temperatures ranging from 10xc2x0 C. to 30xc2x0 C. excluding 25xc2x0 C. according to the present invention, the fluctuation field thus measured is multiplied by (298/T) (where T is the absolute temperature), and the product is taken as a fluctuation field Hf at 25xc2x0 C.
In accordance with the conventional method, a Cr under-layer was first formed on a mirror-polished disk made of Nixe2x80x94P electroless-plated Alxe2x80x94Mg alloy, and then a CoCrTa magnetic layer together with a protective carbon film was formed thereon to fabricate a magnetic disk. The Cr under-layer, the magnetic layer, and the protective layer were formed by Ar-gas sputtering. In this case, the substrate temperature and the Ar pressure were 300xc2x0 C. and 2.0 millitorr, respectively. Further, the Cr under-layer, the magnetic layer, and the protective layer were 50 nm, 25 nm, and 10 nm thick, respectively. The composition of the CoCrTa magnetic layer is Co: 80%, Cr: 16%; Ta: 4%, expressed by atomic %. This composition will be expressed as CoCr16Ta4. The coercivity Hc and the remanence coercivity Hr were 1645 and 1655 oersteds, respectively. Further, the fluctuation fields of magnetic viscosity at 25xc2x0 C. at the field strength equal to the coercivity and at the field strength equal to the remanence coercivity were 13.5 and 13.2 oersteds, respectively. Thus, the fluctuation fields of magnetic viscosity at 25xc2x0 C. at the field strength equal to the coercivity and at the field strength equal to the remanence coercivity exhibit substantially the same value; hereinafter these are called simply the fluctuation field in this specification.
Incidentally, the measuring time of the fluctuation field ranged from 0 to 30 minutes.
A permalloy head having a gap length of 0.4 xcexcm and a coil of 24 turns was used to record magnetic data on the medium, and a magneto-resistive permalloy head was used to reproduce the data in order to examine the electromagnetic transducing properties. The flying height at the time of recording and reproducing data was 80 nm. As a result of measurement, noise at a longitudinal bit density of 150 kFCI (kilo Flux Change per Inch) was 22 xcexcVrms.
Although a magnetic disk unit having a recording density of 300 megabits/square inch could be fabricated by using this medium, a magnetic disk unit having a recording density of 1-gigabit/square inch could not be fabricated.
An object of the present invention is to provide a magnetic recording medium and a magnetic recording and reproducing apparatus suitable for reducing noise at the time of reproducing data and for high-density recording.
FIG. 1 is an enlarged sectional view of a magnetic recording medium embodying the present invention. In FIG. 1, reference numeral 1 denotes a nonmagnetic substrate of Nixe2x80x94P-clad aluminum, Nixe2x80x94P-clad aluminum-magnesium alloy, glass carbon, or the like; 2, a nonmagnetic under-layer for controlling the crystal orientation and crystal grain size of a magnetic film, which is a metallic layer of Cr, Crxe2x80x94Mo, Crxe2x80x94W, Crxe2x80x94Ti, Crxe2x80x94V, or the like; 3, a ferromagnetic thin film of a cobalt-based alloy such as Coxe2x80x94Crxe2x80x94Ta, Coxe2x80x94Crxe2x80x94Pt, Coxe2x80x94O, Coxe2x80x94Ni, Coxe2x80x94Cr, Coxe2x80x94Mo, Coxe2x80x94Ta, Coxe2x80x94Ni-Cr, Coxe2x80x94Nixe2x80x94O, or the like; and 4, a protective lubricant layer in which a carbon film, an oxide film, a plasma polymerized film, fatty acid, perfluorocarbon carboxylic acid, perfluoropolyether, or the like may be used as a single or composite material. A ferromagnetic thin film for use as the magnetic layer 3 is desirably such that the fluctuation field of magnetic viscosity at 25xc2x0 C. at the field strength equal to the remanence coercivity or the coercivity is not less than 15 oersteds, the coercivity is not less than 2000 oersteds, and the thickness of the magnetic layer 3 is not less than 5 nm and not more than 30 nm. It is more desirable that the fluctuation field of magnetic viscosity at 25xc2x0 C. at the field strength equal to the remanence coercivity or the coercivity is not less than 20 oersteds. The ferromagnetic thin film is desirably a cobalt-based ferromagnetic thin film containing at least one element selected from the group consisting of Cr, Ta, Pt, Ni, Mo, V, Ti, Zr, Hf, Si, W, and O, for example, a thin film containing cobalt of Coxe2x80x94Crxe2x80x94Ta, Coxe2x80x94Crxe2x80x94Pt, COxe2x80x94O, Coxe2x80x94Ni, Coxe2x80x94Cr, Coxe2x80x94Mo, Coxe2x80x94Ta, Coxe2x80x94Nixe2x80x94Cr, CO-Ni-0, or the like.
A specific method for measuring the fluctuation field is as follows.
In order to obtain a fluctuation field A, a magnetic field of xe2x88x9210,000 oersteds is applied to a specimen 7 mm square cut out of a magnetic disk before being subjected to dc-erase. Subsequently, a positive magnetic field slightly lower than the coercivity or remanence coercivity is applied to the specimen to obtain time t until the magnetization or remanent magnetization decreases to zero. While the positive magnetic field applied after the dc-erase is lowered gradually, the operation above is repeated. The fluctuation field A is found from the dependence of the coercivity or remanence coercivity on the field applied time thus determined according to Eq. (3). The fluctuation field found from the dependence of the coercivity on the field applied time shows substantially the same value as that of the fluctuation field found from the dependence of the remanence coercivity on the field applied time. Because of measurement simplicity, the fluctuation field A was found from the dependence of the remanence coercivity on the field applied time according to the present invention. A vibrating sample magnetometer of DMS (Digital Measurement Systems) Co. was employed for the measurement purposes. The measuring temperature was at 25xc2x0 C. and the field applied time after the dc-erase was in a range of 0 to 30 minutes.
Data from 8 seconds up to 30 minutes was used when the fluctuation field was found since an error in the applied time tends to become greater in a region of a short time less than several seconds.
Although the magnetic disk was an object in the example above, the present invention is also effective for magnetic recording media such as magnetic tapes.
When a ferromagnetic thin film whose fluctuation field of magnetic viscosity at 25xc2x0 C. at the field strength equal to the remanence coercivity or the coercivity is not less than 15 oersteds and whose coercivity is not less than 2000 oersteds is used, and a magnetic layer 3 whose thickness is not less than 5 nm and not more than 30 nm is used, it is possible to lower the noise level and to raise the S/N value since the cluster size can be decreased at the time of magnetic reversal.
By the combination with a magnetic head using a metal magnetic film in part of the magnetic pole, the medium capable of fast recording is allowed to demonstrate its performance, so that a large-capacity recording and reproducing apparatus can be provided.